Upgrade to workspace 3.0

 - drop support for Cloud9
 - re-create the app using `yo loopback:example`
 - update README

The new project structure (and the updated README) is viewable in the [upgrade-to-workspace-3.0 branch](https://github.com/strongloop/sls-sample-app/tree/feature/upgrade-to-workspace-3.0). I hope the updated example app will make the new project layout more clear.

/to @raymondfeng Please review.
/to @crandmck I guess you can move some parts of README to Confluence: [Using the LoopBack sample app](http://docs.strongloop.com/display/LB/Using+the+LoopBack+sample+app). Let's land this PR first though.
/cc @altsang @ritch